1. Field of Invention
The present invention relates to the coupling of radiation from a resonator and more particularly to the coupling of optical radiation from an unstable laser resonator. The invention herein described was made in the course of or under a contract or subcontract thereunder, with the Department of the Air Force.
2. Description of the Prior Art
Various recognized authorities have been providing inventions and publications dealing with the general understanding and physics of unstable laser resonators for about ten years. Some of the early work which has been accepted and recognized in the field includes the publications entitled Unstable Optical Resonators For Laser Applications, A. E. Siegman, Proc, IEEE, Vol. 53, page 277, March 1965 and Unstable Optical Resonator Loss Calculations Using The Prony Method, A. E. Seigman and H. Y. Miller, Applied Optics, Dec. 1970, Vol. 9, No. 12, pages 2729-2736. Although these materials are merely representative of the state of a developing art, they provide much of the basic understanding which exists with respect to unstable laser resonators. Another early publication recognized by the technical community is Properties Of A Laser With An Unstable Resonator, Anan'ev et al, Soviet Physics, JETP, Vol. 28, No. 1, January 1969, pages 69-74; additional related papers by Anan'ev and others were published mainly in Soviet Physics in the late 60's and early 70's. U.S. Pat. No. 3,641,458 entitled Mode Selective Laser With Small Feedback Reflector And Diffraction Coupled Output, issued to J. T. La Tourrette et al on Feb. 8, 1972 is indicative of the patent art of this era.
One of the characteristics common to all the unstable resonator geometries described and analyzed in the various published papers is the inability to achieve a large fraction of the output power in the central maximum of the far field radiation pattern which is derived from such resonators. This occurs because the existing coupling techniques produce an output beam which is an annulus in cross section. An annular beam having an outer diameter D and an inner diameter d has a far field radiation pattern which contains a smaller fraction of the total beam in the central maximum than does a fully illuminated beam having the diameter D. In general, as the ratio of the outer (D) to inner (d) diameters of the annulus approaches unity, the fraction of the power removed from the resonator which appears in the central lobe of the far field decreases. One approach to increase the size of this fraction is to design the unstable resonator with a large value of geometric magnification (M) where M is equal to the ratio of the annulus diameters (D/d). However, as M is increased, the fraction of the power in the resonator coupled therefrom increases concomitantly resulting ultimately in a reduction in the optical flux in the optical cavity. This phenomenon is referenced to as overcoupling and while it achieves the primary objective of increasing the fractional power in the center of the far field pattern of the output beam, the total power which appears in the far field is reduced because the overcoupling causes a lowering of the optical flux in the optical cavity. These effects and others are analyzed and reported in such papers as W. F. Krupke and W. R. Sooy, Properties Of An Unstable Confocal Resonator CO.sub.2 Laser System, IEEE Journal of Quantum Electronics, QE-5, page 575, December 1969; R. J. Freiberg, P. P. Chenausky and C. J. Buczek, An Experimental Study of Unstable Confocal CO.sub.2 Resonators, IEEE Journal of Quantum Electronics, QE-8, page 882, December 1972; Siegman et al, Unstable Optical Resonator Loss Calculations Using The Prony Method identified above, and A. E. Siegman, Stabilizing Output With Unstable Resonators, Laser physics, May 1971, pages 42-47. Thus, while unstable resonators have been studied and used for a considerable length of time, their use is sometimes limited because for a given resonator they either provide an amount of power in the far field which is less than is desired or the percent of the power occurring in the central maximum in the far field is less than is desired.